The present invention relates to photographic light-sensitive silver halide emulsions wherein the silver halide grains are spectrally sensitized to near infrared radiation at wavelengths above 700 nm with a J-band type sensitizing dye of a particular class of cyanine dyes and to photographic elements and film units employing these emulsions.
It is well known in the photographic art that the photosensitive response of silver halide emulsions can be extended to longer wavelengths by the addition of spectral sensitizing dyes, notably cyanine dyes. This technique has been employed to sensitize silver halide emulsions to a specific wavelength region in the visible and also the infrared portion of the electromagnetic spectrum and has been widely used in the production of photosensitive elements for color photography which comprise a plurality of spectrally sensitized emulsion layers that respond to different wavelength regions of the spectrum. This technique also has been employed in the production of panchromatically sensitized emulsions, generally by employing a combination of sensitizing dyes to provide the requisite sensitivity over the wavelength range of about 400 to 650 nm.
Various cyanine dyes have been used to spectrally sensitize photographic light-sensitive silver halide emulsions, for example: (1) symmetrical and unsymmetrical cationic cyanine dyes obtained from derivatives of 6-fluorobenzothiazole, see Kiprianov and Yagupolsky in J. Chem. USSR, 20, 211: Eng. Trans. 2187 (1950); (2) a spectral sensitizing dye having an amidinium ion auxochrome and numerous cyanine dyes including symmetrical and unsymmetrical polymethine dyes of fluoro-substituted benzothiazoles, see U.S. Pat. No. 3,955,996; (3) unsymmetrical cyanine dyes useful as green sensitizing dyes which possess a benzoxazole nucleus and a 5-fluorobenzothiazole nucleus, see U.S. Pat. No. 4,387,155; (4) pentamethine cyanine dyes of 5-fluorobenzothiazole derivatives useful as the infrared sensitizing dyes above 800 nm, see U.S. Pat. No. 5,254,455; and (5) a rigidized pentamethine dye, see U.S. Pat. No. 5,415,978.
In addition, combinations of two or more cyanine dyes have also been used to spectrally sensitize photographic light-sensitive silver halide emulsions, for example:
(1) U.S. Pat. No. 3,632,349 (issued Jan. 4, 1972) discloses a spectrally sensitized silver halide photographic emulsion whose spectral sensitivity in the red region is raised by supersensitization, i.e., the combination of at least two kinds of sensitizing dyes represented therein by formula (I) and (II), respectively; see column 1, lines 74-75. The dye of formula (I) therein J-aggregates and a suitable spectral sensitivity distribution may be given; see column 3, lines 28-29. By contrast, the dye of formula (II) therein, which may have a furyl group at the number 9-carbon of the dye (see column 2, line 44) and must have at least one sulfo-substituted alkyl group on the resonating terminal nitrogen atom in the heterocyclic nucleus (see column 2, lines 69-70), shows a very weak spectral sensitizing action when used alone, see column 2, line 75 to column 3, line 2; and
(2) U.S. Pat. No. 5,508,161 (issued Apr. 16, 1996) discloses a photographic silver halide photosensitive material which includes an infrared sensitive layer which is spectrally sensitized with a combination of at least two J-band type sensitizing dyes so as to have maximum spectral sensitivity of at least 700 nm; see column 4, lines 12-13.
The benefits of the invention of aforementioned U.S. Pat. No. 5,508,161, e.g., high sensitivity in the infrared region (see column 3, line 63), are obtained only when two or more J-band type sensitizing dyes are combined, but not achieved when J-band type sensitizing dyes are used singly; see column 5, lines 37-43. These patentees state that only a few J-band type sensitizing dyes having a maximum absorption wavelength of 700 nm or longer are known (see column 5, lines 48-55) and, that, after making extensive investigations of the art on J-band type sensitizing dyes having a maximum absorption wavelength of 700 nm or longer, they decided to utilize a combination of dyes rather than a single sensitizing dye to attain the desired sensitization, i.e., at least 700 nm or longer wavelength; see column 5, lines 56-59.
Although the known sensitizing dyes referred to above have generally provided suitable speed and stability at the desired wavelengths; nevertheless, the sensitizing dyes of choice for above 700 nm sensitization have routinely imparted instability and undesirable photographic speed to the sensitized photographic system. Therefore, additional research is necessary to find a solution to this stability problem without compromising the speed of and extent of sensitization by these dyes of choice.
Accordingly, the present invention provides a class of J-band type sensitizing dyes having maximum absorption wavelength above 700 nm to achieve the desired sensitization. More particularly, the present invention provides photographic light-sensitive silver halide emulsions wherein the silver halide grains are spectrally sensitized to near infrared radiation at wavelengths above 700 nm with a J-band type sensitizing dye of a particular class of cyanine dyes resulting in suitable speed, extent of sensitization and stability when used in photographic systems.